Current driver having voltage transition failure-based short circuit protection circuit

Abstract

A current driver has a short circuit protection circuit which monitors the magnitude of the current driver's output voltage. The protection circuit looks for the failure of the output voltage to either change to a prescribed non short-circuit representative value within a prescribed time window after the onset of a voltage transition at the input node, or to maintain that value as dictated by the input signal. If either of these conditions occurs, the protection circuit takes action to reduce the driver's output current to a relatively small `short circuit` current.

What is claimed: 
     
       1. A current driver circuit comprising: an input node to which an input signal, which transitions from a first input signal level to a second input signal level, is applied;   an output node from which an output signal, which normally transitions from a first output signal level to a second output signal level in accordance with said input signal transitioning from said first input signal level to said second input signal level, is derived;   a controllable current supply circuit which controllably supplies an adjustable magnitude current to said output node; and   an output signal transition sensing circuit, which is coupled to said output node and to said controllable current supply circuit, and which causes said controllable current supply circuit to supply current of a first current level to said output node in response to the application of said input signal to said input node and, in response to said output signal having failed to transition from said first output signal level to said second output signal level upon the expiration of a prescribed period of time subsequent to the application of said input signal to said input node, causes said controllable current supply circuit to supply current of a second current level to said output node, said second current level being less than said first current level, but of a level sufficient to maintain current flow from said controllable current supply circuit to said output node.   
     
     
       2. A current driver circuit according to claim 1, wherein said controllable current supply circuit comprises a first controllable current source which controllably supplies a first current of a first magnitude to said output node, and a second controllable current source which controllably supplies a second current of a second magnitude to said output node, said second magnitude being larger than said first magnitude, and wherein said output signal transition sensing circuit has an output coupled to said second controllable current source, and controls said second controllable current source to supply said second current to said output node in response to the application of said input signal to said input node, but, in response to said output signal having failed to change from said first output signal level to said second output signal level upon the expiration of said prescribed period of time subsequent to the application of said input signal to said input node, causes said second controllable current source to terminate its supply of said second current to said output node, while leaving said first controllable current source turned-on and thereby supplying said first current of said first magnitude to said output node, said first magnitude corresponding to said second current level, which is sufficient to maintain current flow from said first controllable current source to said output node. 
     
     
       3. A method of controlling the operation of a current driver circuit which controllably supplies an adjustable level drive current to an output node in accordance with an input signal applied to an input node, said input signal transitioning from a first input signal level to a second input signal level, said output node providing an output signal, which normally transitions from a first output signal level to a second output signal level in accordance with said input signal transitioning from said first input signal level to said second input signal level, said method comprising the steps of: (a) in response to said input signal transitioning from said first input signal level to said second input signal level, causing said current driver circuit to supply a drive current of a first current level to said output node; and   (b) in response to said output signal having failed to transition from said first output signal level to said second output signal level upon the expiration of a prescribed period of time subsequent to said input signal transitioning from said first input signal level to said second input signal level, causing said current driver circuit to supply drive current of a second current level to said output node, said second current level being less than first current level, but of a level sufficient to maintain current flow from said current driver circuit to said output node.   
     
     
       4. A method according to claim 2, wherein said current driver comprises a first controllable current source and a second controllable current source, said second controllable current source being operative to supply a second current having a second magnitude larger than a first magnitude of a first current controllably supplied by said first controllable current source, and wherein step (b) comprises causing said first and second controllable current sources to supply said first and second currents to said output node in response to said input signal transitioning from said input signal level to said second input signal level and, in response to said output signal at said output node having failed to transition from said first output signal level to said second output signal level upon the expiration of said prescribed period of time subsequent to said input signal transitioning from said first input signal level to said second input signal level, causing said second controllable current source to terminate its supply of said second current to said output node, leaving current flowing from said first controllable current source to said output node.